1. Field of the Invention
The present invention relates to a dynamic foil gas bearing that includes a hollow cylindrical member with a rotatable shaft inserted therein and a group of foils mounted on an inner circumferential surface of the hollow cylindrical member, for supporting the rotatable shaft in a journal direction.
2. Description of the Related Art
Generally, centrifugal compressors are used in the art as superchargers for efficiently supplying compressed air. For example, a centrifugal compressor is used as an auxiliary for supplying an engine with compressed air or an auxiliary for supplying a fuel cell with compressed air as an oxygen-containing gas.
It has been desired that central compressors of the type described above rotate at a considerably high speed. Japanese Laid-Open Patent Publication No. 2001-295836 discloses a foil fluid bearing for use as a high-speed bearing.
As shown in FIG. 8 of the accompanying drawings, the disclosed foil fluid bearing comprises a hollow cylindrical stationary holding member 2 nonrotatably fixed in position around the outer circumferential surface of a journal 1, and a foil assembly 3 disposed in an annular clearance between the inner circumferential surface of the stationary holding member 2 and the outer circumferential surface of the journal 1. The foil assembly 3 includes a top foil 4 disposed at a radially inner position, a middle foil 5 disposed radially outwardly of the top foil 4, and six bump foils 6 disposed radially outwardly of the middle foil 5.
The top foil 4 comprises a flat sheet curved into a substantially hollow cylindrical shape and has an end secured to an upper portion of the inner circumferential surface of the stationary holding member 2 by a welded region 4a. The middle foil 5, which also comprises a flat sheet curved into a substantially hollow cylindrical shape, has an end secured to the inner circumferential surface of the stationary holding member 2 by a welded region 5a near the welded region 4a. 
The top foil 4 extends clockwise from the end secured by the welded region 4a toward the other end thereof. The middle foil 5 extends counterclockwise from the end secured by the welded region 5a toward the other end thereof. The top foil 4 has an outer circumferential surface and the middle foil 5 has an inner circumferential surface. The outer circumferential surface of the top foil 4 and the inner circumferential surface of the middle foil 5 are held in sliding contact with each other as sliding surfaces 7.
The top foil 4 includes a crank-shaped portion or angularly bent portion extending from the welded region 4a to the sliding surface 7 thereof, and the middle foil 5 also has a crank-shaped portion or angularly bent portion extending from the welded region 5a to the sliding surface 7 thereof. While the foil fluid bearing is in operation, the bump foils 6 are elastically deformed toward the inner circumferential surface of the stationary holding member 2 under an air pressure developed when the journal 1 rotates about its own axis. At this time, the top foil 4 is deformed about the crank-shaped portion, tending to form an edgy corner. When such an edgy corner is formed, the outer circumferential surface of the journal 1 is brought into sliding contact with the edgy corner and unduly wears the top foil 4.